Method of manufacturing a circuit board

ABSTRACT

A method of manufacturing a circuit board which may include the steps of forming a circuit board with horizontal and vertical fiberglass fibers, rotating the circuit board, and cutting the circuit board so that the horizontal and vertical fiberglass fibers form a non-right angle with a cut line of the circuit board. The circuit board may have a plurality of conductive traces located thereon which pass by areas of higher fiberglass-to-resin material and lower fiberglass-to-resin material to assist in reducing differential to common mode conversion between signals in the plurality of conducive traces.

This is a division of application Ser. No. 11/727,279 filed 26 Mar.2007, now U.S. Pat. No. 7,676,917 which is a division of applicationSer. No. 10/641,070 filed 15 Aug. 2003, now U.S. Pat. No. 7,459,200 thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to circuit board design. Moreparticularly, the present invention relates to improving circuit boardsto improve electrical performance thereof.

Circuit boards, which include printed circuit boards or PCBs, are commonin the industry for electrically connecting components in a single unit.The most common type of printed circuit board is made of a materialreferred to in the art as FR4. This type of circuit board is relativelyinexpensive to manufacture. It is made up of a rectangular grid or clothof fiberglass fibers that are typically bonded to a copper substrateusing an epoxy resin. Also, the fiberglass cloth can be impregnated withthe epoxy resin. A fire retardant (FR) is added to the board, so thatthe board may be subjected to high-heat environments (e.g., whensoldering components to the board). Electrical conductors (e.g., made ofcopper) are deposited in or on either side of the board and transmitsignals between components (e.g., integrated circuit chips) on theboards, sockets for the insertion of other circuit boards (e.g. adaughter card inserted into a motherboard), etc. In the art, thematerial used for FR4 circuit boards has been effective at reducingcrosstalk between signal lines.

FR4 circuit boards are also used to transmit signals in differentialpoint to point interfaces. In such an interface, two signal line tracesare provided to transmit one data signal. A first one of the tracestransmits a part of the data signal that is 180° out-of-phase with theother part of the data signal transmitted on the second one of thetraces. At the receiving end of the two signal line traces, the twoout-of-phase signals are differentiated so as to recreate the originaldata transmitted in the signals. Signal trace pairs are often used inbus architectures.

FR4 has been successfully used for the transmission of differentialsignals at current operating frequencies. As components, includingprocessors, increase in operating frequency, problems may occur in theuse of FR4 for the circuit board material. For example, as busfrequencies increase over 1 GHz (gigahertz or 1 billion cycles persecond), differential to common mode conversion may become a problem fora signal trace pair. In this conversion, the phase difference betweenthe first and second traces changes from the optimal 180° describedabove. If the phase difference drops below a certain threshold, thereceiving device will be unable to differentiate the two signals toretrieve the original data signal. Though such conversion occurs in FR4circuit boards, the level of conversion has not been a factor at lowerfrequencies. At higher frequencies, this conversion may become severe,negating the use of known FR4 circuit boards.

One solution to this problem is to replace the FR4 board with adifferent, and perhaps homogenous, material. Such boards are known inthe art, but are generally more expensive than the common FR4 material.

In view of the above, there is a need for an improved circuit boardmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the use of FR4 circuit boardwith a rectangular lattice structure and a pair of signal traces.

FIG. 2 is a schematic representation of the use of FR4 circuit boardwith a zigzag or herringbone lattice structure and a pair of signaltraces.

FIG. 3 is a schematic representation of the use of modified FR4 circuitboard and a pair of signal traces according to an embodiment of thepresent invention.

FIGS. 4 a-c are schematic representations of an embodiment of thepresent invention for manufacturing a circuit board using portions of anFR4 circuit board.

DETAILED DESCRIPTION

Referring to FIG. 1, a representation of a portion of an FR4 board witha pair of signal traces is shown. The circuit board 10 includes verticaland horizontal fiberglass fibers 11 a, 11 b, respectively. Though onerectangular lattice structure is shown, multiple such lattice structuresmay be provided in the circuit board 10. The circuit board 10 alsoincludes a resin 12, such as an epoxy resin, which is disposedthroughout the rectangular lattice structure of the fiberglass fibers 11a-b. In the board of FIG. 1, a pair of signal line traces 13 a, 13 b areprovided. In this example, the signal lines are generally parallel tothe horizontal fiberglass fibers 11 b. These signal traces may beembedded into the resin, disposed on top of the resin of the circuitboard, etc. As seen in FIG. 1, the first signal line 13 a is generallydisposed between adjacent horizontal fibers 11 b. In this example eachsignal line trace has a width of approximately 5 mils (i.e., 5milli-inches or 0.005 inches). The second signal line 13 b, however, isgenerally disposed over one of the horizontal fibers 11 b. The spacingbetween the traces is 5 mils in this example.

When using the first and second signal line traces for differentialsignaling, it is desirable to have these traces be equivalent in termsof characteristic impedance and propagation constant. With low signalfrequencies, the effect of the non-homogeneous material of the FR4circuit board is negligible on these features of the signal traces. Assignal frequency increases, however, differences in the materialcomposition near the signal traces has a considerable effect on thesefeatures. At high signal frequencies, these features will have an effecton the magnitude and phase of the signals transmitted by the trace. Ifthe circuit board material affects the characteristic impedance and/orpropagation constant in these traces differently, then determining thedifference between the signals in these traces becomes more difficultand could result in loss of the data desired to be transmitted.

In the example of FIG. 1, the board material near the second trace 13 bhas a relatively high ratio of fiberglass-to-resin material, while theboard material near the first trace 13 a is made up of a much lowerratio of fiberglass-to-resin material. At a signal frequency of 604 MHz(i.e., 604 Million Hertz or 604 Million cycles-per-second), a 5 mil widetrace of an FR4 circuit board has a dielectric constant E_(r) thatvaries between 3.32 and 3.50. With two signal line traces having a widthof 5 mils and a spacing of 5 mils, it is estimated that total modeconversion of differential to common mode (i.e., complete signal loss)will occur at 3.5 GHz for 30-inch trace lengths due to the accumulatedphase shift between the pair.

According to an embodiment of the present invention, a circuit board isfabricated using the same types of materials contemplated for FR4circuit boards. A representation of this embodiment is shown in FIG. 2.Referring to FIG. 2, in the portion of the circuit board 20 shown, firstand second fiberglass fiber sets 21 a, 21 b overlap each other. Thesesets form a zig-zag or herringbone design in this embodiment. In otherwords, each fiberglass set is characterized by straight line segmentsconnected by vertices (e.g., as in a periodic, triangular wave-form).Though two sets of fiberglass fibers are shown, one or more such setsmay be provided. As in the FR4 circuit board, a resin is added to formthe circuit board structure. As with the FR4 circuit boards known in theart, there will be areas where the ratio of resin material to fiberglassmaterial will be relatively high (e.g., at area 25) and areas where thisratio will be relatively low (e.g., at area 27). Again, two signal linetraces 23 a, 23 b are disposed in or on the circuit board 20. As seen inFIG. 2, the circuit board material residing near the signal line traces23 a, 23 b is a mixture of different ratios between the resin andfiberglass materials. The effect, in this embodiment, is that thematerial near the first signal line trace 23 a will be similar, overall,in content compared to the material near the second signal line trace 23b. Since the materials near first and second signal line traces 23 a, 23b are similar, overall, there is less differential to common modeconversion when using these traces for differential mode signaling.

In the embodiment of FIG. 2, the fiberglass material is formed in azig-zag or herringbone design. The spacing between adjacent fibers maybe similar to that of the fiberglass lattice of FIG. 1—approximately 5mils. As signal frequencies increase and signal trace width decreases,the density (i.e., thickness and spacing) of the fiberglass materiallattice may be adjusted so as to reduce differential to common modeconversion in signal line pairs.

The rectangular lattice for the fiberglass cloth used in the circuitboard 10 of FIG. 1 is made in a conventional manner. The cloth is madeby taking fiberglass fibers and weaving them together or interleavinghorizontal fibers and vertical fibers. To make the zig-zag orherringbone design of FIG. 2 may be implemented by modifying systemsthat currently manufacture fiberglass cloth. For example, in creatingthe first set 21 a of fiberglass fibers, the apparatus emitting thefiberglass fibers can be moved side-to-side as the fibers are laid downonto a surface. Alternatively, the surface upon which the fibers arelaid can be moved side-to-as the fibers are laid down. Once the zig-zagor herringbone fiberglass fiber is made, it can be adhered to a coppersubstrate with an epoxy resin as with standard FR4 manufacturingmethods. Accordingly, signal line traces 23 a, 23 b may be formed byremoving unwanted copper from the circuit board in a conventionalmanner.

The embodiment of the present invention shown in FIG. 2 may be made at alow-cost similar to conventional FR4 circuit boards that are currentlyavailable. The manufacture of the board of FIG. 2 may use much of thesame equipment as is used in the manufacture of conventional FR4 circuitboards.

Another embodiment of the present invention is shown in FIG. 3. In FIG.3, an FR4 circuit board is made in a conventional manner. In otherwords, a fiberglass cloth of horizontal and vertical fiberglass fibers31 a, 31 b are provided in an epoxy resin 33. After the fiberglassfibers and resin are formed together the entire apparatus is rotated andthen cut to size. For example, in FIG. 3, the fiberglass lattice isrotated 45° (i.e., either the horizontal fiberglass fibers or thevertical fibers form a non-right angle of 45° with cut line). Whenplacing two conductive traces 35 a, 35 b onto the circuit board 37, itcan be readily seen that both conductors pass by areas of higherfiberglass-to-resin material and lower fiberglass-to-resin material,thus assisting in reducing differential to common mode conversionbetween signals in the two conductive traces.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention. Furthermore, certain terminology has been used for thepurposes of descriptive clarity, and not to limit the present invention.The embodiments and preferred features described above should beconsidered exemplary, with the invention being defined by the appendedclaims.

For example, one or more conventional FR4 boards may be rotated (e.g.,by 45°) and sliced into a number of squares. The individual squares fromthe board(s) can then be rotated again to desired orientations andreformed together. An example of this is shown in FIGS. 4 a-c. In FIG. 4a, an FR4 board is rotated 30° and sliced into a number of sections orsquares 41. In FIG. 4 b, a second FR4 board is rotated 45° and slicedinto a number of squares 42. The individual squares can then be rotatedif desired, and then reformed together to form a new FR4 board withconductors 43 a, 43 b.

Though the embodiment of FIGS. 4 a-c is shown with rectilinear shapes,other shapes may be used. Also, as shown in FIGS. 4 a-c, the density ofthe fiberglass fibers need not be the same between the sections used tocreate the new FR4 board.

What is claimed is:
 1. A circuit board comprising: a resin; first andsecond fiberglass fibers; and first and second signal line tracescapable of transmitting electrical signals, wherein a ratio betweenfiberglass and resin material near the first signal line trace issimilar to a ratio between fiberglass and resin material near the secondsignal line trace, wherein the first and second fiberglass fibers form anon-right angle with a cut line of the circuit board, and wherein thefirst and second signal line traces are capable of transmittingelectrical signals at one gigahertz and above.
 2. The circuit board ofclaim 1, wherein the resin is an epoxy resin.
 3. The circuit board ofclaim 1, wherein the first and second signal line traces cross near thefirst and second fiberglass fibers in a diagonal fashion.
 4. The circuitboard of claim 1, wherein the first and second signal line traces crossnear the first and second fiberglass fibers in a zig-zag fashion.
 5. Thecircuit board of claim 1, wherein the first and second signal linetraces pass by areas of higher fiberglass-to-resin material and areas oflower fiberglass-to-resin material to assist in reducing differential tocommon mode conversion between signals on the first and second signalline traces.
 6. The circuit board of claim 1, wherein the first andsecond signal line traces are a pair of differential signal line traces.7. The circuit board of claim 1, wherein the first and second signalline traces are to be used for differential mode signalling.
 8. Thecircuit board of claim 1, wherein the ratio between fiberglass and resinmaterial near the first signal line trace is an overall ratio betweenfiberglass and resin material near an entire portion of the first signalline trace and wherein the ratio between fiberglass and resin materialnear the second signal line trace is an overall ratio between fiberglassand resin material near an entire portion of the second signal linetrace.
 9. A circuit board comprising: a resin; first and secondfiberglass fibers; and first and second signal line traces capable oftransmitting electrical signals, wherein the first and second fiberglassfibers diagonally cross near the first and second signal line traces,wherein a ratio between fiberglass and resin material near the firstsignal line trace is similar to a ratio between fiberglass and resinmaterial near the second signal line trace, wherein the first and secondfiberglass fibers form a non-right angle with a cut line of the circuitboard, and wherein the first and second signal line traces are capableof transmitting electrical signals at one gigahertz and above.
 10. Thecircuit board of claim 9, wherein the resin is an epoxy resin.
 11. Thecircuit board of claim 9, wherein the first and second signal linetraces pass by areas of higher fiberglass-to-resin material and areas oflower fiberglass-to-resin material to assist in reducing differential tocommon mode conversion between signals on the first and second signalline traces.
 12. The circuit board of claim 9, wherein the first andsecond signal line traces are a pair of differential signal line traces.13. The circuit board of claim 9, wherein the first and second signalline traces are to be used for differential mode signalling.
 14. Thecircuit board of claim 9, wherein the ratio between fiberglass and resinmaterial near the first signal line trace is an overall ratio betweenfiberglass and resin material near an entire portion of the first signalline trace and wherein the ratio between fiberglass and resin materialnear the second signal line trace is an overall ratio between fiberglassand resin material near an entire portion of the second signal linetrace.
 15. A circuit board comprising: a resin; first and secondfiberglass fibers; and first and second signal line traces capable oftransmitting electrical signals; wherein the first and second fiberglassfibers cross near the first and second signal line traces in a zig-zagpattern, wherein a ratio between fiberglass and resin material near thefirst signal line trace is similar to a ratio between fiberglass andresin material near the second signal line trace, wherein the first andsecond fiberglass fibers form a non-right angle with a cut line of thecircuit board, and wherein the first and second signal line traces arecapable of transmitting electrical signals at one gigahertz and above.16. The circuit board of claim 15, wherein the resin is an epoxy resin.17. The circuit board of claim 15, wherein the first and second signalline traces cross near the first and second fiberglass fibers in adiagonal fashion.
 18. The circuit board of claim 15, wherein the firstand second signal line traces pass by areas of higherfiberglass-to-resin material and areas of lower fiberglass-to-resinmaterial to assist in reducing differential to common mode conversionbetween signals on the first and second signal line traces.
 19. Thecircuit board of claim 15, wherein the first and second signal linetraces are a differential pair of signal line traces.
 20. The circuitboard of claim 15, wherein the first and second signal line traces areto be used for differential mode signalling.
 21. The circuit board ofclaim 15, wherein the ratio between fiberglass and resin material nearthe first signal line trace is an overall ratio between fiberglass andresin material near an entire portion of the first signal line trace andwherein the ratio between fiberglass and resin material near the secondsignal line trace is an overall ratio between fiberglass and resinmaterial near an entire portion of the second signal line trace.